1. Field of the Invention
The present invention relates to an optical scanning apparatus used in a write system of a digital copying machine, a laser beam printing machine or the like. More particularly, the present invention is concerned with a multibeam scanning apparatus capable of simultaneously scanning a scanned surface such as a photosensitive member with multiple light beams so that the recording rate can be increased.
2. Description of the Related Art
Conventionally, a method for increasing the revolution speed of a polygonal mirror is known as means for increasing the recording rate of the optical scanning apparatus used in the write system. However, the above method results in an increased load on a motor for driving the polygonal mirror. The increased load degrades durability of the optical scanning apparatus, and causes noise and vibration. Eventually, improvement in the recording rate is limited.
A multibeam scanning apparatus has been proposed which simultaneously records a plurality of lines with a plurality of light beams formed simultaneously. For example, Japanese Laid-Open Patent Application No. 6-331913 discloses that light beams emitted by a light source made up of a plurality of semiconductor laser diodes are combined by means of a beam splitter. However, the proposed method needs a fine control of the optical axes of the light beams. Such a fine control needs a complex adjustment mechanism and cumbersome adjustment work.
Another multibeam scanning apparatus has been proposed which as a multibeam light source, in which a plurality of semiconductor laser diodes and a coupling lens are incorporated into a module and beams emitted by the laser diodes are made closer to each other and are then combined by a beam combining means. The above multibeam light source has a simple structure that can be assembled with ease, and makes it possible to easily adjust a sub scan pitch.
The above-mentioned method using the laser diodes and the beam splitter has a disadvantage in that a housing or optical components forming the light sources may be deformed due to an environment change. Such a deformation results in a variation in the attitude of the optical unit itself, and a positional displacement between the semiconductor lasers and the coupling lens. Thus, the sub scan pitch on the scanned surface is liable to be varied. The above disadvantage needs a mechanism for measuring the sub scan pitch, and another mechanism for correcting the sub scan pitch based on the sub scan pitch thus measured. Particularly, Japanese Laid-Open Patent Application No. 6-331913 shows a feedback correction in which a prism is used for each beam in order to finely adjust the optical axis and thus cause the light beams to travel in the correct direction. However, such a feedback correction is implemented by a complex structure and is thus expensive.
In contrast, the proposal using the module provides the following advantages. The semiconductor laser diodes and the coupling lens are integrally supported by an identical base member, and the beams are emitted in the main scanning direction at given angular intervals. With the above structure, it is possible to adjust the sub scan pitch by merely adjusting the revolution of the entire light source unit and assemble the apparatus easily. In this regard, the semiconductor laser diodes may be arranged close to each other in order to omit the beam combining means.
However, in any proposal using the modules, the beam emitting positions are offset from the optical axis (rotational axis). Thus, the adjustment of the sub scan pitch varies the beam emitting points, so that the light beams may be inclined. It is thus required to adjust the optical axis with a high precision sufficient to hold down a fine revolution.